Magnetic apparatus to control the energy distribution of an electron beam



Oct. 6, 1970 w. w. HUNT. JR v3,532,913

MAGNETIC APPARATUS T0 CONTROL THE ENERGY DISTRIBUTION OF AN ELECTRONBEAM I Filed April 10, 1968 INVENTOR. WILL/,4 Ml/(J/Y? Mr.-

,wJ. MUM fml wwiwr. & WW0; A W m L 7//////////////A P A if b M v m m rif Unitecl States Patent 3,532,918 MAGNETIC APPARATUS TO CONTROL THEPBIEIQIIRIGY DISTRIBUTION OF AN ELECTRON William W. Hunt, Jr.,Chelmsford, Mass., assignor to the United States of America asrepresented by the Secretary of the Air Force Filed Apr. 10, 1968, Ser.No. 720,264 Int. Cl. Hillj 29/46 US. Cl. 313-83 2 Claims ABSTRACT OF THEDISCLOSURE An apparatus for using the energy dependence of the electronreflection coeflicient to produce an electron beam having a narrowerenergy distribution than the original source beam by applying either amagnetic field alone or prependicular electric and magnetic fields toforce the electrons to undergo successive collisions with a reflectingsurface.

CROSS-REFERENCES TO RELATED PATENTS A patent application of even dateherewith entitled Apparatus To Control The Energy Distribution Of AnElectron Beam by William W. Hunt, in, has been filed in the US. PatentOfiice bearing Ser. No. 720,263 with a filing date of Apr. 10, 1968which, for purposes of distinction, has been termed Non-MagneticReflection-Type Electron Velocity Filter. The invention described hereinhas been described as a Magnetic Reflection-Type Electron VelocityFilter. The Non-Magnetic Reflection- Type Electron Velocity Filtergeometrically forces the electrons of a beam to undergo successivecollisions with a reflecting surface by positioning the beam at apredetermined angle in relationship to the reflecting surface in orderto provide a substantially narrower energy distribution than that in theoriginal beam. The Magnetic Reflection-Type Electron Velocity Filterprovides a substantially narrower energy distribution by applying eithera magnetic field alone or perpendicular electric and magnetic fields toforce the electrons of the beam to undergo successive collisions with areflecting surface.

BACKGROUND OF THE INVENTION This invention relates to apparatus forcontrolling the energy distribution in an electron beam, and moreparticularly, to an apparatus using the energy dependence of theelectron reflection coeflicient to produce an electron beam having anarrower energy distribution than the original source beam.

Certain scientific research apparatuses require highly monoenergeticelectron beams, such as in ionization, col-- lision, and electronattachment experiments and electron diffraction devices. Monoenergeticelectron beams also find applications in electron-beam devices such aselectron microscopes, cathode-ray tubes, and various display devices(e.g. TV picture tubes, radar display tubes) whose resolving power isnow limited by the energy/velocity spread in the electron beam. Thisinvention solves the problem of how to produce low energy beams withvery narrow energy/velocity distributions. The low-energy monochromaticbeams produced by this invention can be accelerated or decelerated towhatever energy is desired.

SUMMARY OF THE INVENTION The reflection coeflicient R of a given surfacefor lowenergy electrons (say for E 35 ev.) is known to depend on theenergy E with which the electrons hit the surface. Also, in this sameenergy range, the reflected electrons retain essentially all of theirincident energy. (See Niedermeyer and Holzl, Phys. Stat. Sol. 11, 651,1965.) The Patented Oct. 6, 1970 is caused to strike a surface then someof the beam will be reflected and some will be absorbed. The reflectedcurrent is given by and the energy distribution of the reflectedelectrons is now R(E)I(E), that is, the number of reflected electronscrossing a plane normal to the reflected beam current with energybetween E and E+dE is R(E)I(E)dE. If the reflected beam is in some wayforced to impinge again on the same surface, then the current after thissecond refiection will be given by and the energy distribution of theelectrons in this now twice-reflected beam is R (E)xI(E). Similarly,after 11, such reflections the current is and the energy distribution isR (E)I(E).

If the reflection coefficient undergoes maxima and minima as theincident energy (velocity) is increased from zero, then the electronenergy (velocity) distribution of a multiply-reflected (diffracted) beamwill have sharply defined peaks at energies (velocities) correspondingto each maxima in R overlapped by the initial energy (velocity)distribution. If the positions of these maxima in R are known inadvance, then the original energy (velocity) distribution can be shiftedby means of accelerating or retarding voltages to give significantoverlap with only one of the maxima. In this case, the energy (velocity)distribution of the multiply-reflected electrons will be sharply peakedat the one energy (velocity) corresponding to this maximum, and thesharpness of the peak will increase as n, the number of reflections, isincreased. Even if R has no maxima the energy (velocity) distribution ofthe reflected electrons will become sharper and sharper at that energy(velocity) corresponding to maximum overlap of R and I(E).

In accordance with the present invention an apparatus exploiting thisselective characteristic of successive reflections may be constructed inthe following way: an appropriate reflecting surface (e.g. a fiat sheetof metal) is mounted in a vacuum system such that the surface isparallel in one direction, X, perpendicular in the other, Y, to anapplied magnetic fieldi.e. so that the normal, Y, to the reflectingsurface is perpendicular to the magnetic field. A surface for applyingan electric field perpendicular to the magnetic field is mountedopposite and parallel to the reflecting surface. Electrons having thedesired intial energy (velocity) distribution are made to impinge on thereflecting surface. The magnetic field causes the reflected electrons tobe curved back to hit the surface again and again-the number ofreflections being determined by the radius of curvature of the es in themagnetic field and by the length of the reflecting surface in thedirection perpendicular to the magnetic field. If an electric field isapplied by making the opposing surface positive with respect to thereflecting surface, then the electrons follow trochoidal rather thancircular trajectories and have their characteristic energy (velocity)distribution only at the plane of the reflecting surface. They travelthe major portions of their trajectories at higher energies so thatspace-charge effects are lessened. In this case, the number ofreflections can be varied by changing the electric field. The beamhaving the narrow filtered energy/velocity distribution is thenextracted in any of a number of ways, the simplest being to allow it topass through a slit (located exactly n times the distance betweensuccessive impacts beyond the point of initial impact) in the reflectingsurface. In practice the n-tirnes-reflected ribbon of monoenergeticelectrons would be scanned to impinge on the center of the slit byvarying the voltage applied to the field-forming plate or by varying themagnetic flux density.

An object of the present invention is to provide an apparatus for usingthe energy dependence of the electron reflection coeflicient to producean electron beam having a narrower energy or velocity distribution thanthe original source beam.

Another object of the present invention is to provide an apparatus tocontrol and narrow the energy distribution of an electron beam byapplying a magnetic field alone to force the electrons of the beam toundergo successive collisions with a reflecting surface.

Yet another object of the present invention is to provide an apparatusto control and narrow the energy distribution of an electron beam byapplying perpendicular electric and magnetic fields to force theelectrons to undergo successive collisions with a reflecting surface.

The various features of novelty which characterize this invention arepointed out with particularlity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,however, its advantages and specific objects obtained with its use,reference should be had to the accompanying drawing and descriptivematter in which is illustrated and described a preferred embodiment ofthe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Description of the drawings FIG.1 is a preferred embodiment of the invention, taken along line 11 ofFIG. 2; and

FIG. 2 is a front view of the preferred embodiment shown in FIG. 1.

Now referring to FIG. 1, there is shown electron beam source 1 havingvariable voltage source 8 connected thereto to provide a variablecontrol of electron beam 2. Electron beam source 1 is conventional andmay be of the type utilized in cathode ray tubes. Electron beam 2 iscomprised of low-energy electrons. In electron beam source 1, there isshown voltage source 14 providing voltage by way of transformer 15 forheating filament 16 which thereupon provides a low energy electron beamemerging from slot 17.

An appropriate reflecting surface 4 is provided by utilizing flat metalsheet 3 which is positioned such that the normal, Y, to reflectingsurface 4 is perpendicular to an applied magnetic field B (everywhereperpendicular to and directed into the plane of the single drawing) thatis provided by current flowing through solenoid 7. Flat metal sheet 3 isgrounded. Flat metal sheet 3 includes input slot 3a and output slot 3b,and its surface opposite source 1 is the reflecting surface 4.

Parallel surface 9 is provided by utilizing flat metal sheet 10. Sheet10 may also be grounded (see below).

Parallel surface 9 is used for applying an electric field perpendicularto the aforementioned magnetic field. The electric field is provided byconnecting the positive side of variable voltage source 11 to metalsheet 10 and grounding the negative side thereof. Variable voltagesource 11 is variable from 0 to the values required for achieving therequisite electric fields. Electron source 1 and metal sheets 3 and 10are mounted in the conventional vacuum systems utilized, for example, incathode ray tubes. Solenoid 7 and variable voltage sources 8 and 11 maybe mounted in the conventional manner externally to the vacuum system.Metal sheets 3 and 10 are positioned in the vacuum system in a mannerconventional to the cathode ray tube technique.

Now referring to FIG. 2, there is shown a view of FIG. 1 showing anapplied magnetic field B provided by solenoid 7 parallel to thereflecting surface in the X direction.

In the operation of the apparatus referring to FIG. 1, electron source 1is positioned so that electron beam 2 is directed through input slot 3a.Electrons having the initial energy (velocity) distributioncharacteristic of source 1 are made to impinge on the reflecting surface4. The magnetic field, parallel to the reflecting surface in the Xdirection, causes the reflected electrons to be curved back to hit thesurface again and again. The number of reflections being determined bythe radius of curvature of the es in the magnetic field and by thelength of the reflecting surface. With an electric field also applied bymaking opposing surface 9 positive with respect to reflecting surface 4,then the electrons follow trochoidal rather than circular trajectoriesand have their characteristic energy (velocity) distribution only at theplane of reflecting surface 4. They travel the major portion of theirtrajectories at higher energies so that space charge effects arelessened. In this case, the number of reflections can be varied bychanging the electric field by varying voltage source 11. The beamhaving the narrow filtered energy/velocity distribution is thenextracted by allowing it to pass through output slit 3b located exactlyn times the distance between successive impacts beyond the point ofinitial impact. In practice the n-tirnes-reflected ribbon ofmonoenergetic electrons would be scanned to impinge on the center ofslit 3b by varying the voltage to the field-forming plate 10 or byvarying the magnetic flux density by varying the current flow throughsolenoid 7, by means of variable current source 12. Solenoid 7 is aconventional source or means for forming a magnetic fiield. In adifferent embodiment, the solenoid may be replaced by a permanentmagnet.

It is noted that the magnetic field mode of operation may be utilizedalone to obtain circular trajectories or the magnetic field incombination with an electric field is used to obtain trochoidaltrajectories. To obtain exclusively magnetic field operation, voltagesource 11 is eliminated or varied to obtain a zero voltage output and toobtain a combination operation of magnetic and electric field, voltagesource 11 is increased to the desired magnitude.

Thus, there has been provided an apparatus wherein the original electronbeam with a wide energy distribution has been controlled to provide aresultant electron beam with a substantially narrower energydistribution by the use of the energy (velocity) dependence of thereflection coeflicient of a surface for slow electrons. This inventionthereby solves the problem of how to produce low energy electrons beamswith very narrow energy/ velocity distribution. The low-energymonochromatic beams produced by this apparatus can be accelerated ordecelerated to whatever energy is subsequently desired.

A constructive test using literature values of R(E) for polycrystallinePt (Niedermeyer and Holzl, Phys. Stat. Sol. 11, 651, 1965) combined withan initial energy distribution corresponding to normal component of a3000 K. thermionic distribution accelerated by 0.1 ev. gave the reducedhalf-widths and attenuations indicated in the following Table I after 0,3, 5, and 7 reflections. (Here W (O)=0.175 ev.) This is a severe testsince neither the energy distribution nor this particular set ofreflection data has maxima of the type discussed heretofore.

TABLE I While in accordance with the provisions of the statutes, I haveillustrated and described the best form of the invention now known tome, it will be apparent to those skilled in the art that changes may bemade in the form of the apparatus disclosed without departing from thespirit of the invention as set forth in the appended claims and that insome cases certain features of the invention may be used to advantagewithout a corresponding use of other features.

Having now described my invention, what I claim as new and desire tosecure by Letters Patent is as follows:

1. Apparatus to control and narrow the energy distribution of anelectron beam comprising only first and s cond parallel surfaces, eachhaving first and second ends, said first surface being a reflectingsurface at ground potential and having an input and output slot at saidfirst and second ends, respectively, a voltage source variable from zeroto a preselected positive voltage magnitude to selectively apply anelectric field between said first and second parallel surfaces varyingfrom zero to a preselected magnitude, an electron beam source providingan initial electron beam of low-energy electrons, said electron beamsource being positioned to permit entry of said initial electron beambetween the parallel surfaces through said input slot, means to apply amagnetic field parallel to and between said first and second parallelsurfaces to force the electrons of said initial electron beam to undergoa predetermined number of successive collisions with said firstreflecting surface along the length thereof to provide a resultant beamof substantially narrower energy distribution than that of said initialelectron beam, said resultant electron beam being emitted at said outputslot,

said voltage source being set at zero for applying said magnetic fieldexclusively between said first and second parallel surfaces and to apredetermined positive voltage for applying a combination of saidelectric and magnetic fields between said first and second parallelsurfaces.

2. Apparatus to control and narrow the energy distribution of anelectron beam as described in claim 1 further including means to varythe intensity of said magnetic field to permit control of the number ofsuccessive collisions of said electrons with said first reflectingsurface.

References Cited UNITED STATES PATENTS 2,061,387 11/1936 Prinz 3132,175,697 10/1939 Nelson 313-84 X RAYMOND F. HOSSFELD, Primary Examiner7 US. Cl. X.R. 313-79, 84

